This invention relates generally to injection molding machines. More particularly, this invention concerns vented injection molding machines having a two-stage screw therein.
An injection molding machine typically receives synthetic resinous material as a particulate feedstock, heats, masticates and plasticates the material to a moldable consistency and forces the plasticated material into a mold. In the past, the presence of readily volatilized substances or moisture in a particulate synthetic resinous material feedstock for an injection molding machine frequently led to defects in a molded product fashioned by the molding machine. Many defects are attributed to the evolution of gaseous pockets containing water vapor or volatilized substances. For example, defects such as polymer degradation and diminished physical material properties in a molded product have been associated with gaseous pockets. Moreover, surface defects, such as loss of gloss and voids, sometimes occur in the molded product as a result of gaseous pockets. These latter defects are sometimes merely cosmetic but do, on occasion, substantially affect physical properties of the molded product. As a result of the increased potential for defective products, moisture and volatilizable substances must be eliminated from synthetic resinous material during molding thereof.
One solution for avoiding moisture in feedstock requires a separate drying apparatus to predry hygroscopic synthetic resinous material and to remove the moisture therefrom before being supplied to an injection molding machine. Provision of separate drying apparatus includes an additional capital expense which detracts from profitability of the injection molding apparatus.
To accommodate a feedstock having readily volatilized substances therein, an injection molding machine should be provided with a vent to allow gaseous volatiles to escape after the feedstock has been plasticated and masticated to a molding consistency. The known prior art vented molding machines, however, are subject to bleed of plasticized material from the vent opening with an associated problem of possible blockage of the vent opening.
When dealing with feedstock having fine particle size, such as a powdered feedstock, a vent opening is also necessary in a molding machine to eliminate air occlusions which may occur from air pockets fed along with the feed material into the plasticating screw of the molding machine.
Examples of particular synthetic resinous materials for which venting is desirable during injection molding are as follows: polyesters, polycarbonates, nylons, acrylics, ABS, styrenic polymers, acetal polymers, polyphenylene oxides, and barrier resins (such as "Barex", and "Lopac").
In fashioning articles of indeterminate length from synthetic resinous materials, vented extruders allow removal of volatiles as well as moisture from feedstock. Extruders, however, operate with essentially steady material flow into the machine, essentially steady or continuous flow of product material out of the machine, and essentially steady flow of material through a vent section of the extruder machine. As a result of the essentially steady operation of the vented extruders, only small perturbations of flow conditions inside the barrel at a vent opening typically occur.
A typical injection molding machine, by comparison, operates in a cycle having a plasticating portion during which feedstock is prepared to a molding consistency, an injection portion during which the plasticated material is injected into a mold cavity, and a holding portion during which material in the mold is permitted to solidify. The cyclical operation of the molding machine causes unsteady flow conditions to prevail inside the molding machine. Such unsteady flow conditions have a contributory effect on the bleed problem described above where a vent opening communicates with a screw receiving bore to exhaust volatile materials.
Moreover, in a vented molding machine a second stage of the screw, downstream of a vent portion, must be relatively long to adequately develop the material pressure necessary to force the screw out of the barrel against a back pressure. This long second stage can lead to the need for an additional support at the usual cantilevered end of the machine barrel as well as other difficulties.
During the injection portion of a molding cycle, a plasticating screw is typically impulsively stroked forward through a distance of several inches to inject a charge of plasticated material into a mold cavity. During this injection portion, pressures in the plasticated material on the order of 20,000 psi have been known to exist and to aggravate the vent bleed problem.
A particularly undesirable result frequently occurs due to bleeding of plasticated material from the vent opening: the plasticated material may solidify in the vent opening and thereby block the vent opening. When this unfavorable result occurs, the subsequent release of volatilized material from the vent opening is severely inhibited, if not actually prevented.
Various injection molding machines have been proposed in the prior art to overcome problems of the type described above. However, each of the prior art machines is objectionable by virtue of having one or more of the following disadvantages: narrow range critical operation; low plasticating capacity; and extensive modifications of a conventional mechanical or hydraulic operating system.
Thus, a need continues to exist for an effective injection molding machine having a vent that permits volatile materials and water vapor to be released from a plasticized material during an injection molding machine cycle.